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Author Topic: STEPS TO TUNE A YAGI from SCRATCH - Print this out -  (Read 7918 times)
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« on: April 11, 2005, 01:27:56 PM »

Hola,

Below is a summary on how to tune a homebrew or modified Yagi for 1:1 swr. I've been working with a group of guys on 6M who are building Yagis, but have found some had confusion and frustration when tuning them and needed help.  Print this out for future reference, as this info applies to ANY frequency...

73,
Tom, K1JJ
------------------------

Hi All,

Many of you are going to be putting up new modified or homebrew 6M Yagis. I wanted to make sure everyone understood the finer details of tuning it for 50 ohms +J0.  A homebrew or modifed design needs some extra steps to adjust the swr flat compared to a store bought "pre-fab".  

Generally, most 6M store bought Yagis will need no driven element TIPS adjustment. Only the T match needs adjustment to give a 50 ohm +J0 match. [50 ohms resistive and no reactance]   This is because the store bought Yagi has already had a prototype built and the exact driven element length dimensions are already determined in the real world. All Yagis have boom currents, boom to element taper influences and other minor things that require the driven element tips to be adjusted the first time out.  There's no getting around this.

So here's how to tune a computer generated new PROTOTYPE Yagi the FIRST time out: [50.135 Mhz is used as this example]


1) MEASURE out a 1/2 wave multiple coax feedline and use the MFJ-259B to determine it is an exact 1/2 wave.  492/ 50.135 mhz = 9.8135'.   X the velocity factor of .66 = 6.47'  for RG-213.  So use a coax TEST feedline  that is 6.47' X1  or  X2  or X3, etc for your test feedline.  To determine it is accurate, short one end and connect the other end to the MFJ and tune the MFJ to find a sharp dip. This dip will read about 1-2 ohms and will show the frequency. Trim off coax to get this frequency near to 50.135 +- 50kc.  VERIFY this or all future readings may be incorrect.

2)  Trim off 1" from each end of the driven element and slot them both 90 degrees with a hack saw. [two cuts and four flaps]  Insert a 4" long,  1/4" dia solid aluminum rod with hose clamps. These are your element tips adjustment.

3) Now with the Yagi about 6' off the ground and in the CLEAR from metal or objects, stand back behind the Yagi reflector and measure the SWR on the MFJ - look at BOTH resistance and reactance. IMPORTANT:  The T match is adjusted ONLY to move the "50 ohms" resistance close to 50 ohms.... and the tips on the elements are used to get the "J0" to J0 reactance reading tuned correctly.  

Moving the T-match around will show anywhere from 20-70 ohms resistance. And, moving the tips in and out will show J -300 to +300   reactance [or whatever].  There is INTERACTION between the two, but you want to end up with 50 ohm resistive and J0 ohm reactance. This is 1:1 swr.  You must include the tips in these intial adjustments - the T match alone cannot do this.

Once the tip lengths are determined, you can add permanent tips and get rid of the hose clamps, but the slide-able tips are OK to leave as is. NOW, in the future, you can build the NEXT Yagi with fixed tips since you know the exact length - just like a store bought Yagi.

The idea is to rock back and forth between the tip length and T match to find a PERFECT match of 1:1.

The parasitic elements do not need any special tuning since the Yagi is usually designed for a wide bandwidth [50-50.3 , etc] and 1/2" error here or there will have virtually no effect on the pattern.

IMPORTANT: As a final test, add a 3' coax jumper [or 1/4  wave, not critical] to the feedline and verify that the swr changes very little. A few ohms is OK. This tells you that match is truly flat, since there is no standing waves on a 1:1 line. If the swr changes significantly, then there is a test set up problem...

This is not as hard as it sounds, but is important to master if you plan on building and tuning Yagis from scratch.  Store bought Yagis are simple to tune in comparison, but nothing like rolling your own and doing a better job on the cheap, caw mawn.

Feel free to pass this email along to anyone else building Yagi antennas...

Thanks.

73,
Tom, K1JJ
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wa1knx
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« Reply #1 on: April 11, 2005, 11:17:42 PM »

hi tom vu,
        nice info!  i dialed in my 3 el 75mtr array similar.  first, i tuned up the
driven element w/o any parasitics for lowest swr with my match. next added both parasitics. i set up a test xmtr a good ways behind the array.
next, i tuned up the parasitics for max FB. next retuned the driven element
for 1:1 swr. next trimmed up the parasitics again for max FB, then retouched the driven array. for 1:1.

        i need to get something up for 6mtrs, esp if i'm out here for the
summer!

73 deano!
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Fred k2dx
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« Reply #2 on: April 15, 2005, 08:57:38 PM »

Bill Orr described the Omega Match and it really saves a lot of trouble. It's a modified gamma match. Two variable capacitors tune the driven element
for resonance and impedance...the gamma arm connection is made somewhat shorter than normal.

For those big boom antennas I came up with a novel twist...I built a 60 foot boom 20 meter 6 element yagi. The element tapers and mounting plates were all computer designed (W2PV method) . The Omega match was built in an outdoor PVC electrical box with the two adjustments made by 1/4 inch aluminum rods along the boom to the mounting point.

With the antenna in place (after a preliminary tuning check at a reachable height) it was raised to 100' and touched up in place with the other antennas on the tower affecting it however. The resonance and match were set precisely with ease and the tuning rods then hose clamped to the boom.

Remotely tuning it was a breeze. I highly recommend reading Bill Orr's beam antenna book, even though it's very dated.

73, Fred   k2dx
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